The invention relates to a selective amplifier stage with a frequency-selective input filter to which there is coupled inductively an amplifier transistor, wherein the input impedance of the amplifier transistor is smaller than the source impedance driving it at the operating frequency.
Such selective amplifier stages are known. The input impedance of the amplifier transistor in such a selective amplifier stage is smaller than the source impedance driving it when the amplifier transistor is operated using a bipolar transistor as amplifier transistor in common base circuit and using a field-effect transistor as amplifier transistor in common gate circuit. In selective amplifier stages wherein the amplifier transistor is operated in common base circuit or in common gate circuit, respectively, with the input impedance of the amplifier transistor consequently being smaller than the source impedance driving it, the coil required for coupling the amplifier transistor to the input filter is not coupled magnetically to the filter coil, and is, therefore, also not arranged together with the filter coil on a common coil carrier. For, the opinion prevails that when the amplifier transistor is operated in common base circuit or in common gate circuit, respectively, it is not possible to use the magnetic coupling of the coupling coil to the filter coil, customary in other basic circuits such as common emitter or source circuits because the low ohmic input impedance of the amplifier transistor is damping the input filter too much in a common base or common gate circuit, respectively, thus lowering the selectivity of the input filter accordingly. On the other hand, however, a magnetic coupling of the coupling coil to the filter coil is desirable because this solution is more space-saving and also more cost-efficient than a non-magnetic coupling of the amplifier transistor to the input filter.